Conventionally, a three-phase alternating current arc furnace in which arcs are produced between a metal material in a furnace shell and three electrodes inserted into the furnace shell to melt the metal material by arc heat is widely used as a melting furnace for melting metal materials such as metal scrap.
In the melting operation of a metal material using such an electric arc furnace, conventionally, there arises a problem of non-uniform melting of the metal material.
In the three-phase alternating current electric arc furnace, three electrodes inserted downwards into the furnace shell are disposed so as to form a triangle around the central axis of the furnace shell in a planar view, that is, disposed so that the three electrodes are respectively positioned at the apices of the triangle.
As a result, in the furnace shell, there exist so-called hot spots at positions with a short distance from the electrodes (that is, close to the electrodes), and so-called cold spots at positions with a long distance from the electrodes (that is, distant from the electrodes).
The metal material at the hot spots is easily melted since the metal material at the hot spots is strongly heated by the electrodes, but the heating by the electrodes is relatively weak at the cold spots. This causes ununiform melting that the metal material at the cold spots remains unmelted even after the metal material at the hot spots is completely melted.
Due to such a ununiform melting, there arise some problems. One problem is, for example, that melting efficiency is worse and the cost for electricity required for melting goes up. Another problem is, for example, that even after the metal material at the hot spots is completely melted, powerful heat is continuously applied during the metal material at the cold spots is continuously being melted, and thus investment of excessive electricity is necessary, melt-erosion of a refractory material of a furnace wall accelerates, and a melt-eroded portion of the refractory material has to be repaired in short cycles.
As a countermeasure against these problems, there has been proposed an electric arc furnace which makes a furnace shell rotate relative to fixed electrodes, or an electric arc furnace which makes electrodes rotate relative to a fixed furnace shell.
For example, the former electric arc furnace is disclosed in Patent References 1 and 2, and the latter electric arc furnace is disclosed in Patent Reference 3.
The electric arc furnace with a rotating apparatus as above can move the position of a metal material originally placed at a cold spot to a hot spot, and a metal material originally placed at a hot spot to a cold spot by rotating the furnace shell relative to the electrodes during melting, and thus the problem of ununiform melting can be remedied.
At this time, it is effective to place the electrode originally positioned in a center region in a circumferential direction of a hot spot into a center region in the circumferential direction of an adjacent cold spot by rotating the furnace shell relative to the electrode by approximately 60° in the circumferential direction.
In order to confirm an in-furnace state of the electric arc furnace during operation, the present inventors have stopped the melting operation once, cooled the furnace and examined the inside thereof, and as a result, they have found that unmelted residue of the metal material is present in the vicinity of a tapping hole or a slag door of the furnace shell.
The cause of the formation of unmelted residue in the vicinity of the tapping hole differs according to the type of a furnace.
FIG. 12A and FIG. 12B are views illustrating the unmelted residue of a metal material in an eccentric bottom tapping electric arc furnace (EBT furnace).
A reference numeral 80 represents an EBT furnace having a furnace shell 82, which has a furnace bottom portion 84 partially protruding outwards from an inner surface of a circular circumferential wall portion 85 of the furnace shell 82 further than an outer surface of the circumferential wall portion 85 in a radial direction to form a protruding portion 86. The protruding portion 86 forms a shelf-like portion with a small gradient, and thereon is formed an opening, that is, a tapping hole 88, which passes therethrough in a vertical direction. The tapping hole 88 is blocked with a cover, which is not illustrated, at the outside on a lower side of the protruding portion 86.
In the EBT furnace 80 with such a configuration, when the metal material is charged into the inside of the furnace shell 82 via a charging opening 95 at an upper end thereof, a part of the metal material may be mounted on the shelf-like protruding portion 86 in the vicinity of the tapping hole 88. It is considered that since the part of the metal material mounted on the protruding portion 86 is positioned with a long distance from electrodes 83 and is weakly heated, the metal material at that part remains unmelted until tapping. A reference numeral 87 in of FIG. 12B illustrates unmelted residue formed in the vicinity of the tapping hole 88.
In the EBT furnace 80, unmelted residue of metal material may be formed in the vicinity of a slag door 91 that is disposed opposite to the tapping hole 88 in the radial direction, in some cases.
The EBT furnace 80 has the slag door 91 that passes through the circumferential wall portion 85 of the furnace shell 82 in an inward and outward direction, and a slag-door bottom portion 92 that extends outwards from the slag door 91 in the radial direction. The slag door 91 may be blocked with a door or the like, but, during a melting operation, external air can infiltrate into the furnace via the slag door 91 (via a gap when blocked with the door), and as illustrated by the arrow in of FIG. 12B, cool air may flow inside the furnace from the slag door 91 toward a dust collection hole 94 attached to a furnace roof 93. For this reason, it is considered that the metal material inside the furnace is cooled in the vicinity of an end portion of the slag door 91 at a side close to the dust collection hole 94 and thus unmelted residue 96 is formed.
On the other hand, also in the case of a spout tapping electric arc furnace having a tapping hole that passes through a circumferential wall portion of a furnace shell in a radial direction and a spout that extends outwards from the tapping hole in the radial direction, the tapping hole is kept in an open state during a melting operation, and thus external air infiltrates into the furnace therethrough. It is considered that, for this reason, due to the same cause as for the slag door 91 in the EBT furnace, a metal material inside the furnace is cooled and remains unmelted in the vicinity of the tapping hole.
Also, in the vicinity of a slag door in the spout tapping electric arc furnace, the metal material remains unmelted similar to the vicinity of the slag door 91 in the EBT furnace.
The tapping hole and the slag door as described above are generally positioned neither in a center region of a cold spot nor in a center region of a hot spot during the operation of the electric arc furnace. Therefore, it is difficult to satisfactorily melt the metal material, which has remained unmelted around the tapping hole or the slag door, by the operation merely switching the hot spot and the cold spot by means of relative rotation between the furnace shell and the electrodes.
Naturally, in the operation of an electric arc furnace which does not perform the switching between hot spots and cold spots by a rotation using a rotating apparatus, it becomes more difficult to satisfactorily melt the metal material which has remained unmelted around a tapping hole or a slag door.
As another countermeasure against the above-described problems, there has been proposed, as illustrated in FIG. 17A and FIG. 17B, that burners 106 are fixedly attached to a circumferential wall portion 104 of a furnace shell 102 to face inwards in the furnace at a position in a center region of each of the cold spots in a circumferential direction and, a metal material M at the cold spots is melted by a flame from the burner 106.
As illustrated in FIG. 17B, when the burner 106 can bring a flame into direct contact with the stacked metal material immediately in front of the burner 106 at an initial stage, the metal material M can be satisfactorily melted. However, in contrast, as illustrated in FIG. 17C, after the metal material immediately in front thereof is melted, the flame from the burner 106 cannot be brought into direct contact with the remaining metal material, and the remaining metal material is heated only by heat in the atmosphere.
For this reason, the heating of the remaining metal material M by the burner 106 becomes weak rapidly, and thus, heating efficiency by the burner 106 for the material at the cold spot is worse, the melting time in the electric arc furnace is prolonged, and a large amount of heating energy is required, and thus, there arises a problem that the total costs go up.
It is possible to expand a heating range by increasing the number of burners; however, in this case, energy cost becomes worse, and thus it has been deemed to be not practical.
Patent Reference 1: JP-A-S60-122886
Patent Reference 2: JP-A-2014-40965
Patent Reference 3: JP-A-H07-190624